Phantom models are used, inter alia, in radiology and radiation therapy for calibration and validation of devices for conducting imaging techniques, for planning of radiation therapy treatment, and for training of medical staff. Therein, the phantom models are supposed to display the properties of human tissue with respect to the interaction with electromagnetic radiation as realistically as possible.
Phantom models are used with methods employing ionizing radiation (x-rays and radioscopy, computer-assisted tomography, therapeutic irradiation), for magnetic resonance tomography (MRT), and in ultrasound.
Therein an image of the phantom model, which is used for example for calibration or for quality control of the device, is generated using the respective imaging technique.
Simple geometric models exist, the purpose of which is not the exact simulation of a specific tissue, but which are primarily supposed to provide reliable material properties. Materials are normally used, which have interaction properties that hardly resemble human tissue.
Moreover, anthropomorphic models, which are supposed to simulate the anatomy, morphology, and tissue properties of the human body in the context of the respective imaging technique, are known in the art. Such models can be equipped with dosimeters for dose measurement and can be combined with simple geometric models.
In particular, anthropomorphic models for ionizing techniques have a broad area of application in dose calibration, validation, and reduction. Measurements using such phantoms verify computer models for therapy planning in radiation therapy. Computer-assisted tomography protocols are established and optimized based on phantom scans.
Currently most computer-assisted tomography users mainly use geometric models for dose calibration and therapy planning. However anthropomorphic models are also used with the aim of more precise calibration and therapy planning compared to geometric models.
Current anthropomorphic models according are made by laborious and expensive methods, wherein materials resembling human tissue are used. From these materials, organs are shaped and assembled, such that a part of the body (i.e. only the upper abdomen), or the whole torso/body can be simulated.
In addition, current anthropomorphic models display the real anatomy and interaction properties with electromagnetic radiation of the simulated body and organs only in an idealized manner. Since the individual sections of the anthropomorphic models are respectively replicated from only one homogeneous material, no heterogeneities, which are typically found in reality, exist within the organ model. Generally, only a limited number of tissues are simulated, for example lung, bone, and soft tissue.
Consequently, images generated by means of anthropomorphic models also deviate substantially from images of real patients. Therefore, calculations for dosimetry, protocol optimization, and device calibration are potentially subject to substantial sources of error. For instance in therapy planning using known phantom models, the exact radiation dose that a patient receives cannot be verified using patient individual phantoms.
An exact calculation of the administered radiation dose is especially important in the area of radiation therapy. In this application very high radiation doses are administered to tumour tissue, whereas surrounding tissue is supposed to be preserved. To this end technically sophisticated systems are available by now. The possibility however to empirically test the actual dose deposition in individual patients is lacking in radiation therapy.
In order to eliminate these uncertainties, to obtain reliable measurement data and to test these empirically, a new generation of realistic phantoms is needed. The current methods of manufacture are unable to display the individual human anatomy accurately. In consequence a completely new approach for the manufacture of realistic models is necessary.
Radiation dense contrast agents are known, the functional principle of which is based on the high atomic number of the elements contained in the contrast agent, whereby a large number of electrons exists around the nucleus, which absorb the incoming electromagnetic radiation. In particular, these contrast agents are used in computer-assisted tomography and x-ray diagnostics.
In a publication, Theodoraku describes printing of geometric shapes and two-dimensional x-ray images of a real patient using a potassium iodide solution as a contrast material on paper (Theodoraku et al., Phys. Med. Biol. 49 (2004), 1423-1438). Subsequently, two dimensional x-ray images of the printouts were generated.
Therein, sheets of paper were printed repeatedly in some cases in order to deposit a larger amount of contrast material. Since every individual sheet allows only a certain amount of printing events, sheets of paper imprinted with the same arrangement of contrast material were stacked on top of each other in some cases in order to achieve a higher radiation density of the model. No three-dimensional structures however were modelled using this method.
3D printing methods facilitate a fast and cost-efficient production of individualized products and therefore are utilized for example for the development of prototypes. The different 3D printing methods have in common that two-dimensional elements are stacked on top of each other in a specific arrangement and thereby give rise to a three-dimensional object. One of these methods is based on stacking of simple printing paper. Thereby, each individual sheet can be imprinted and for example cut by a laser, such that the desired shape and colour of the object is generated in the end.